Friction reduction by poly(n-alkyl methacrylates) in solution and dry films



United States Patent m US. Cl. 252-12 2 Claims ABSTRACT OF THEDISCLOSURE A poly (n-alkyl methacrylate) wherein the alkyl groupcontains 22 carbon atoms which reduces the friction of solid surfaceswhen applied in solution or as a dry film.

Reference is hereby made to patent application, Ser. No. 642,673, ofHenry Gisser and Marco Petronio, for Fri:- tion Reduction by Copolymerof N-alkyl Methacrylates and Methacrylic Acid in Solution, filed May 25,1967, assigned to the same assignee of this patent application.

This invention relates to lubricants, and more particularly relates tomethacrylate polymers for reducing the friction properties of slidingsurfaces when subjected to the lubricants.

The use of polymers as oil additives has been practiced for many years,but only recently has attention been given to the relationship of thepolymer to the lubrication process. Early observations indicated thatpolymer containing motor oils reduced engine friction, i.e., lowerengine friction was obtained with polymer containing oils thannon-polymer containing oils of the same viscosity This was attributed tothe reduction of viscosity of polymer containing fluids at high shearrates which prevails during engine operation, The temporary reduction ofviscosity of polymer solutions under high shear is well known. Theseearly observations on reduced engine friction with polymer containingoils were confirmed when it was determined that the polymer containingoils also had a minimum in the friction-viscosity curve. The polymercontaining oils also yielded less engine wear at viscosities below thosecorresponding to minimum friction and above these viscosities wearremained low. These observations are consistent with the classicallubrication picture in which the minimum corresponded to transitionbetween mixed boundary and fluid film lubrication. In several instances,polymers have also been employed as dispersants in lubricating oils.

Therefore, up to the time of the instant invention, the action ofpolymers in friction reduction compounds has been limited to that of oiladditives primarily for increasing solubility or effecting viscosity. Ithas been discovered, however, through a study of the friction behaviorof polymer solutions in the boundary region in which surface phenomenahave been isolated from volume phenomena, that polymer moleculesthemselves are capable of enhancing the friction properties on a solidsurface. The friction behavior of polymeric compounds in the boundaryregion is dependent upon the interactions of the polymer with the solidsurface and hence, as would be expected, is a function of the chemicalstructure of the polymer molecule and its conformation on the surface.This invention therefore is indicative of the strong influence of thechemical structure of polymers on friction and wear behavior.

In the past, considerable work has been done on the friction behavior oflow molecular weight organic compounds in solution and in the dry stateon solid surfaces, nevertheless, little work has been done with polymersolutions. The low molecular Weight organic compounds al- 3,496,103Patented Feb. 17, 1970 though instantaneously successful in increasingthe friction reduction properties of surfaces have proved extremelyunsuccessful in retaining this beneficial effect for any significantperiod of time. This failure of an essential lubrication property is dueto the inherently poor wear resistant film formed on the surface.

It is an object of this invention to provide a new friction reducingcompound.

Another object of this invention is to provide polmeric frictionreducing compounds.

A further object of our invention is to provide a friction reducingcomposition having good wear resistant properties.

In accordance with the present invention, it was found that thepoly(n-alkyl methacrylate)s when the alkyl consists of a carbon chainhaving 1 to 22 carbon atoms, and preferably 14 to 22 carbon atoms,significantly reduces the friction on a solid surface when used eitherin solution or as films deposited by evaporation of solvent. Thepolymers were prepared thru emulsion polymerization, although variouspolymerization techniques can be used. The molecular weights of thevarious polymers are given in Table I.

TABLE L-MOLECULAR WEIGHTS OF POLY(n-ALKYL METHACRYLAIES) RelativePolymer 1 Determined by membrane osmometry. 2 Relative to poly (methylmethacrylate).

Coefficient of friction was determined on a modified Bowden-Lebenmachine in which the tangential force was detected by strain gagesmounted on a strain ring, the signals being amplified and recorded. Allfriction measurements were made with a A in. diameter 440C stainlesssteel ball rider on an approximately 6 mm. thick plane substrate. Steel,copper, and glass substrates were employed for the measurements. Thesteel substrate was 1020 steel plate prepared by rinsing in boilingbenzene, polishing successively with 400, 600, 2/0, 3/0, and 4/0 siliconcarbide paper and rinsing in boiling benzene. It was then placed in adesiccator over anhydrous calcium sulphate for 24 hours before use. Thecopper substrate (99.98% Cu) was prepared in the same manner as thesteel. The glass specimens were cut from 6 mm. thick polished plate,silvering quality soda lime glass, abraded under water successively with400 and 600 silicon carbide paper followed by rinsing with hot distilledwater. These specimens also were stored over anhydrous calcium sulphatefor 24 hours before use. All measurements were made at a gram load and0.04 cm./sec. sliding speed. The length of traverse was 1.3 to 1.9 cm.for each measurement. Polymer solutions were made with benzene andxylene of spectrographic grade, although various petroleum hydrocarbonfluids can be employed. Three drops of polymer solution were placed in acontinuous line along the path to be traversed by the rider, the firstdrop being placed at a location so that it wet the rider. The areacovered by the fluid was approximately 0.9 square centimeters. Frictionmeasurements were started 5 to 10 seconds after applying the polymersolutions. Unless otherwise indicated all measurements were made at apolymer concentration of 1.0 mg./ml. The standard deviation of thecoeflicient of friction measurement was 0.005.

All the polymers yielded considerable reduction in friction from thesolvent alone. However, as the side chain increased in length, there wasa progressive reduction in the coefficient of friction. There are minordifferences in behavior with the different substrates. For example, with1020 steel, there was first a rapid reduction from the initial valuewhich then reached a minimum in the neighborhood of 16 carbon atoms.With copper the total reduction of coefiicient of friction from themethyl to the docosyl was small but significant, whereas with glassthere was a uniform reduction with increasing length of the side chain.While these individual differences are probably associated withdifferences in intersection of the polymer with the particularsubstrate, the similar behavior on all substrates indicates that thegeneral conformation of the polymers on the substrates is approximatelythe same insofar as friction is affected. However, whatever theparticular conformation, the length of the alkyl group is a significantfactor in reducing the coefficient of friction on the surface.

It should be noted that the concentration of the polymer in xylene,benzene or other solvents may vary from 0.01 to 10.0 mg./ ml. withoutany significant effect on the coefficient of friction.

Table 11 gives the data for sliding the 440C stainless steel rider onthe substrates with a xylene solution of the polymer. The data reportedare the kinetic coefficients of friction. The coefiicients of frictionfor the dry glass, copper and steel were 0.85, 0.39, and 0.36,respectively. TABLE II.COEFFIC1ENT OF FRTC-TION (It) WITH 01% SOLUTIONOF POLY (n-ALKYL METHAC'RYLATES) 1N XYLENE USING A STAINLESS STEEL RIDERp on Steel 11 on Copper a on Glass 0. 215 O. 257 O. 232 0. 200 O. 250 0.225 0. 190 0. 250 0. 217 0. 179 0. 244 0. 202 O. 170 0. 253 0. 187 0.150 0. 230 0. 156 l). 150 0. 243 0. 141

As stated previously, the relationship of the polymer to friction andwear phenomena is also dependent on the interaction of the polymer withthe solid surface. This interaction is primarily an adsorptionphenomenon. The polymer conformation on the surface is best described bystating that most, but not all, of the carbon chain or vinyl backbone ofthe polymer lies fairly close and substantially parallel to the solidsurface secured thereto by adsorption of the polar groups; i.e. theester moiety, in the side chains. The alkyl groups of the side chain areoriented normal to the surface, and this orientation to the surfacetaken together with close packing in condensed layers, leads to a filmwhich effectively prevents metal-on-metal contact, and thus leads tofriction reduction. The remainder of the polymer carbon chain form loopsoriented away from the surface and extending into the solution.

Further tests were conducted to determine whether the structuralfeatures which led to friction reduction with polymer solutions gave thesame effect with dry films. Accordingly, friction measurements were madeon glass and copper substrates with films deposited from 1.0 mg/ml.solutions of the poly(n-alkyl methacrylate)s in xylene. Substratesurfaces with several drops of solution were permitted to remain in airfor several hours until excess solvent had evaporated, then placed in adesiccator over anhydrous calcium sulphate for 24 hours beforemeasurement, The calculated thickness of the dry films was approximately3500 A. whereas the estimated thickness of the films adsorbed fromsolution in the presence of sol- A STAINLESS STEEL RIDER ON COPPER ANDON GLASS a on Copper p on Glass The anti-wear properties ofpolyoctadecyl methacrylate were shown in the following experiment: 1%solution of the polymer was prepared in a petroleum hydrocarbon fluid,naphthenic base (viscosity of 20 centistokes at 37.5C). Wear scarmeasurements in a four-ball apparatus obtained with these solutions wereconsiderably lower (0.240 millimeter average) than corresponding wearscar diameters for the petroleum fluid having no polymer additives(0.334 millimeter average).

While the particular compositions and methods of application describedherein are well adapted to meet the objects of the present invention,various modifications or changes may be resorted to without departingfrom the scope of the invention as defined in the claims.

We claim:

1. A method of applying a dry friction reducing film for a solid surfacewhich comprise the steps of (a) applying a 1.0 mg. per ml. solution ofpoly(n-alkyl methacrylate) in a solvent selected from the groupconsisting of xylene and benzene on to said solid surfaces wherein thealkyl group of said poly(n-alkyl methacrylate) has 22 carbon atoms, anda molecular weight about 1,590,000,

(b) allowing the filmed surface to remain in air until excess solventhas been evaporated therefrom, and

(c) placing the evaporated filmed surface in a moisturefree atmosphereprior to use.

2. A polymeric friction reducing composition consisting essentially of0.1-10 mg. of poly(n-alkyl methacrylate) per ml. of a solvent, saidsolvent being selected from the group consisting of benzene and xylene,said poly(nalkyl methacrylate) having a molecular weight about 1,590,000and wherein the alkyl group of said poly(n alkyl methacrylate) has 22carbon atoms.

References Cited UNITED STATES PATENTS 2,091,627 8/1937 Bruson 252-562,330,773 9/1943 Zimmer et al 252-56 2,407,954 9/1946 Fenske et al 252562,917,375 12/1959 Hudson 44-62 2,927,013 3/1960 Lowe et a1. 44623,037,955 6/1962 Carmen et al 1l7148 X 3,287,264 11/1966 Topper 25256 XDANIEL E. WYMAN, Primary Examiner W. CANNON, Assistant Examiner US- Cl-252--56v

